1. Field of the Invention
The present invention relates to a transmitter apparatus having a linearizing circuit to compensate for distortion produced in a transmission power amplifier that is required to have a high linearity, and more specifically, to a transmitter apparatus in radio communications equipment at microwave frequencies ranging up to the millimeter band, having a linearizing circuit to compensate for distortion produced in a transmission power amplifier thereof.
2. Description of the Related Art
In radio communications using microwave frequencies ranging up to the millimeter band, combined amplitude and phase modulation techniques such as 16 QAM and 64 QAM are often used to transmit multiple bits per symbol. To obtain satisfactory performance with those modern modulation techniques, a high linearity is required in a transmission power amplifier for radio communications equipment.
As illustrated in FIGS. 23(A) and 23(B), ordinary power amplifiers will exhibit some gain reduction and phase shift as their operating level comes close to the saturation power level. Phase shift that appears in their outputs may be either a lead as in FIG. 23(B) or a lag as in FIG. 23(A). Such non-linear characteristics of power amplifiers in a near-saturation range will introduce two kinds of distortion with their outputs. One is amplitude modulation distortion (hereafter "AM-AM distortion") and the other is amplitude-phase modulation distortion (hereafter "AM-PM distortion"). A conventional method to prevent amplifier outputs from being distorted is to set the operating level of the power amplifiers much lower than their saturation level. Since this conventional method forces the power amplifiers to have a high power output only for safety margin, the amplifier design results in larger sizes, greater power dissipation, and higher production costs.
To solve this problem, Japanese Patent Laid-open Publication No. 53-82110 (1978), for example, discloses a system that allows a power amplifier to operate in its near-saturation range by applying a linearizing circuit to offset the effect of distortion generated therein. FIG. 24 is a block diagram of such a conventional linearizing circuit.
This linearizing circuit is employed in an intermediate frequency (IF) or radio frequency (RF) circuit, which is located before the final power amplification stage. The circuit generates a counter-distortion signal component whose phase is shifted by 180 degrees from that of the real distortion signal component expected to be produced in the power amplifier, so that it will cancel the distortion.
More specifically, the main signal input is distributed to three signal paths (a), (b), and (c) by two hybrids 101 and 102. On the signal path (b), a distortion generator 103 having a non-linear device causes some distortion with the main signal. On the signal path (c), an attenuator 104 and a phase shifter 105 are disposed to adjust the amplitude and phase of the main signal so that their output will be equal in amplitude but 180.degree. out-of-phase to the main signal on the signal path (b). Through a process of combining the two output signals of the paths (b) and (c) with a hybrid 106, the main signal component is eliminated and only the distortion component remains isolated. For this process, the distortion generator 103 has to be adjusted previously so that the isolated distortion component will have an adequate amplitude but inverse phase to cancel the distortion component expected to be generated in the final power amplifier. Finally, the main signal component from the signal path (a) and the distortion signal component from the hybrid 106 are combined together by a hybrid 107, while the phase difference between them is adjusted by a delay line 108.
However, such a conventional linearizing circuit is too complicated in structure and requires large-scale and costly hardware. The miniaturization and cost reduction of the power amplifier is accompanied by a large and costly linearizing circuit and, therefore, it does not directly contribute to the miniaturization and cost reduction of the transmitter equipment as a total system.